Retired Maj Gen Julian Thompson, who led 3 Commando Brigade during the Falklands war, said the act was “totally unforgivable” but said a lenient sentence would be appropriate.

However, other figures said clemency would send out the wrong message and increase the risk of British military personnel being executed or mistreated by insurgents.

Maj Gen Thompson told BBC Radio Four’s Today programme: “I understand that he’s quite an experienced guy. People are talking about him being battle hardened and therefore there is no excuse.

“Well actually it’s the other way round, the more times you do tours in that filthy war, the more pressure there is on you.”

His comments were echoed by retired Colonel Mike Dewar who said society “had to make exceptions for soldiers in extraordinary circumstances”.

He told BBC Breakfast: “These soldiers have had their officer killed - their platoon commander, a very fine young man - they had seen their friends brutalised - lost legs - they had been under the pressures of battle for weeks and months.

“I think you have to understand this is a completely different environment to a cold-blooded murder in normal circumstances.

“This was murder - have no doubt about it, and I make no excuses - but I think society does have to make some exceptions for soldiers in these extraordinary circumstances.”

But Julian Lewis, the Conservative MP for New Forest East and former shadow defence minister, said Marine ‘A’ should not be given special treatment.

“Whilst I have the greatest respect for the opinions of Julian Thompson who is a very highly regarded military leader, the problem with calling for and showing leniency and tolerance in the case of a war crime is that it increases the risk to our personnel if they fall into enemy hands,” said Mr Lewis.

“It is an atrocity but it is also a betrayal of British personnel because it will increase the risk to them - we are fighting enemies at the moment who are already operating outside the general laws of war.”

Marine ‘A’ was convicted after avideo of the killing, captured on a helmet camera, was found on a comrade’s laptop.

As the man convulsed on the ground, Marine A told him: “It’s nothing you wouldn’t do to us.”

He then turned to others on his patrol and said: “Obviously this doesn’t go anywhere, fellas. I just broke the Geneva Convention.”

Two other Marines, ‘B’ and ‘C’, who were alleged to have been party to the killing were acquitted after the two and a half week court martial.